1. Field of the Invention
This invention relates to a device for recording a coherent image in a multimode optical cavity.
Storage of optical information in a memory is provided in a number of different forms and finds potential applications in many different fields. Among these can be mentioned the recording of two-dimensional or three-dimensional objects for safeguard purposes. Photography is the first form of optical storage but lasers have given rise to the development of new methods which are applicable in particular to the audiovisual and data-processing fields. However, although many researches have been made and commercial outlets at the present time are limited, two applications give every evidence of representing the possibilities offered by optical storage: these are holographic storage and storage of television images on video-disk.
The field of the invention is that of storage and subsequent processing of coherent images. In contrast to the two applications mentioned above, however, the specific field under consideration is that of dynamic recording of coherent images.
The device of the invention has the design function of storage and processing of an optical image based on the use of photosensitive electrooptical crystals. The operation of the device is based on energy transfer properties in dynamic holography experiments with photorefractive crystals.
2. Description of the Prior Art
A device of the prior art described in French patent application No. 82 09448 filed on May 28th, 1982 permits storage of a coherent image in a multimode optical cavity. This device comprises a coherent source which generates a signal wave and a reference wave, optical guiding means for forming a closed loop in which each signal wave circulates, an interactive amplifying medium which is placed on the path of the signal wave and on that of the reference wave. The signal wave interferes with the reference wave of said medium in order to form a diffraction grating and thus to receive energy from this reference wave. The guiding means form a multimode optical cavity, the interactive amplifying medium being placed in the Fourier plane of this cavity. A semitransparent plate is placed on the path of the signal wave in order to extract part of the energy of the multimode optical cavity.
However, this device for image storage in a cavity is attended by the following disadvantage: in this configuration which makes use of a crystal, the optical path relating to each mode of the cavity must remain equal to a multiple of this wavelength. This phase condition which corresponds to oscillation entails the need for practically perfect stigmatism for the formation of the image within the cavity.
The device in accordance with the invention makes it possible to overcome this disadvantage. In fact, it permits circulation of an optical image within a cavity by producing a transfer of information alternately from a first crystal to a second crystal. This transfer is produced simply by synchronization of two shutters.
Thus in the device of the invention there no longer exists any condition in regard to the phase since the information is stored alternately in the first crystal and then in the second crystal. The resulting effects on the constructional design of the device are highly significant. In fact, there are no longer any precise tolerances to be observed in regard to the position and quality of the optical components and lenses used in the construction of the cavity.